Effect of the external refractive index on fluorescence kinetics of perylene in human erythrocyte ghosts

Fluorescence kinetics of perylene molecules in hemoglobin-free human erythrocyte membranes is investigated as a function of the refractive index of the external medium Varied by adjusting the concentration of sorbitol or sucrose in an aqueous suspension of erythrocyte ghosts. It has been found that the fluorescence of perylene in erythrocyte ghosts decays nonexponentially, with the mean decay time decreasing from 7.13 to 5.70 ns with an increase in the refractive index of the suspension from 1.333 to 1.442. An analysis of the dependence made it possible to obtain an estimate of the second-rank orientational order parameter of perylene in the human erythrocyte membrane [P-2 (cos theta)] = 0.32... 0.43, which bears witness of considerable ordering of perylene molecules along acyl chains of phospholipids constituting the membrane. Good correspondence of the order parameter with the value of the steady-state emission anisoptropy of perylene in erythrocyte ghosts suggests that acyl chains of phospholipids in the human erythrocyte membrane are predominantly oriented along the normal to its surface.